The present invention relates to a plastics metering pump, comprising two rotors which are coupled to each other by means of gearwheels and can be driven in opposite directions and which are mounted in a pump housing provided with suction connection and outlet connection, wherein each rotor has a rotor shaft, the rotor shaft ends of which are supported in the walls of the pump housing.
Metering pumps are known in all sizes and construction types. As plastics metering pumps are known, in particular, manually operated piston pumps, as are known on soap dispensers for liquid soaps or, as here particularly of interest, also in the hotel and catering industry, where, for instance in fast food outlets, mustard, ketchup or coffee cream are dispensed in metered volume with such manually operated piston pumps. Despite these metering pumps, the dispensed quantity varies relatively strongly however, since, in the metering pumps, in particular of the kind just described, the stroke path should actually be fully utilized with each actuation, yet this is generally not the case. Instead, one, two or three short strokes are often performed and the quantity accordingly varies very strongly. As long as this quantity is dispensed merely as the accompaniment to a hamburger, this is of only minor importance. However, where such metering pumps are also used to add a specific quantity of a liquid food to a recipe, the taste is varied by incorrect actuation, which is not always appreciated by the customers.
Although various different pumps are perfectly well known, in particular including rotor pumps, these are mostly designed as relatively high-precision metering pumps made of metal, and this is also necessary in the food industry, where large quantities have to be dispensed in metered volume. For commercial application, however, mostly very cheap disposable metering pumps are provided, generally free of charge. Accordingly, such metering pumps must be made of plastic, have a structure which is as simple as possible, and work reliably.
The plastics metering pump which is here of interest should in particular be conceived for foods which are dispensed in so-called tubular bags, or other flexible packagings made of plastics sheets.
Many liquid foods also contain relatively large solid components. Typical examples of liquid foods of this kind are, for instance, tartare sauce, mustard sauces with pickles, vanilla sauce with chocolate and almond slivers, etc. With the currently standard metering pumps, liquid foods of this kind cannot be dispensed in metered volume. In particular with so-called gear pumps, as one such is represented, for instance in FR-2313971, this is scarcely realizable. In the case of larger solid particles, such as, for instance, almond sticks, these are ground by the rotors or block the rotors. Accordingly, for such metering pumps, in particular metering pumps in which the rotors have two-bladed or multi-bladed rolling elements, enter into consideration. Examples of such pumps are known from U.S. Pat. No. 3,054,417, where a metering pump for liquid mediums for the admixture of further liquids is shown, wherein in this case each rotor has three impellers and these impellers roll one against another and thus transport the medium onward. In such pumps, between the housing and the individual rotor blades there is sufficient space to transport also liquids with solid parts. Here the larger solid parts are less a problem than, in fact, the smaller solid parts, which remain stuck to the rotor blades rolling mutually one against another and in the course of the rolling process, are completely crushed, whereupon a coating can form, which coating reduces the discharge capacity and can even lead to blockages.
The same also applies to a metering pump of WO 95/24556, in which only two-bladed rotors are represented, but which rotors likewise roll mutually both one against another and against the housing wall.
A further rotary piston pump is known from EP-1 892 417. This is conceived, however, as an insert for an outer metallic housing, but is created for single use and has a housing made of plastic. The toothed gear with which the correct relative position of the two rotors is ensured is a component part of a gearing disposed outside the actual pump and not a component part of the parts provided for single use. Although the rotors, which intermesh during operation, have concave recesses, these are not shaped in such a way that the rotary piston pump is particularly suitable for products with solid components. In particular, the comparatively tight radii of the concave recesses allow deposits to develop precisely in these regions, which deposits remain in the pump and, in the case of foods, possibly quickly perish through contact with the outside air.